1. Field of the Invention
The present invention relates to a DC brushless motor, and more specifically to a DC brushless motor preferably used to drive blades designed for stirring in a heating chamber.
2. Description of the Related Art
An AC (Alternating Current) motor has been used as a motor to drive blades designed for stirring in a heating chamber of a microwave oven. A temperature in a heating chambers of some microwave ovens is increased to a maximum of 300° C. to 500° C. The motor to drive the blades designed for stirring in the heating chamber of the microwave oven is used in an environment where heat is easily transmitted from the heating chamber to the motor, because a shaft of the motor is arranged to project into the heating chamber of the microwave oven, and the motor is attached to a side wall of the heating chamber. In general, electronic components have heat-resistant temperature of about 180° C. at the highest, and can therefore not be used in an environment where heat in the heating chamber is immediately transmitted thereto. In this regard, the AC motor is suitable for use in a high temperature environment because the AC motor does not need to have electronic components mounted thereon. On the other hand, it has been difficult to use a DC (Direct Current) brushless motor in such an environment. Accordingly, in order to use the DC brushless motor as the motor to drive the blades designed for stirring in the heating chamber of the microwave oven, it is necessary to provide a cooling mechanism in the microwave oven or the DC brushless motor.
Concerning a cooling mechanism of the DC brushless motor, an outer-rotor brushless motor for use in an office automation appliance is disclosed in JP 07-245925 A. This motor includes a rotor, a stator, a motor shaft, a bearing portion, a cup-shaped bracket, and an attachment base. In FIG. 4 of JP 07-245925 A, a rotor frame 7 includes a plurality of rising portions 7a defined in a top plate of the rotor frame. Once the rotor frame is caused to rotate in a direction indicated by an arrow in FIG. 4 of JP 07-245925 A, outside air is sucked into the rotor frame by a fan and the rising portions defined in the top plate of the rotor frame to blow the air to windings, a core, and a housing inside the rotor frame, whereby the windings, the core, and the housing are cooled.
In recent years, microwave ovens with higher added value have been demanded. Specifically, a reduction in the probabilities of uneven cooking and uneven temperature has been demanded. In addition, an increase in the number of cooking modes has been demanded to make it possible to employ a variety of methods of cooking. In order to achieve the reduction in the probabilities of the uneven cooking and the uneven temperature, a fine adjustment of a rotation rate of the blades designed for stirring in the heating chamber and shifting between normal rotation and reverse rotation of the blades designed for stirring in the heating chamber are demanded. In addition, in order to increase the number of cooking modes, a capability to set a plurality of patterns of the rotation rate of the blades designed for stirring in the heating chamber is demanded. Accordingly, there is a demand for use of a DC brushless motor capable of fine control as the motor to drive the blades designed for stirring in the heating chamber.
However, in the case where the DC brushless motor described in JP 07-245925 A is used in the microwave oven, heat from the heating chamber tends to be easily transmitted to a circuit board because an end portion of the DC brushless motor on the circuit board side, i.e., an upstream side of air flow, is arranged inside the heating chamber. Therefore, electronic components mounted on the circuit board may become damaged. It is therefore impossible to adopt this DC brushless motor as the motor to drive the blades designed for stirring in the heating chamber of the microwave oven.